In a pneumatic tire, for example, in a tire used for a passenger car, as the vehicle speed becomes higher, the frequency of abrupt starting and abrupt stopping becomes high. As a result, a strong fitting force between bead parts and the rim is required. On the other hand, the use of tubeless tires is increasing for high speed running, and in such tubeless tires, it is mounted on a rim with a particularly strong fitting force. In a tire for a bus or a truck exposed to a heavy load, as the load increases, it comes to be fitted to the rim with a stronger fitting force.
When assembling such a tire to the rim (c), as shown in FIG. 9, the bead bottom surface (b) of the bead part (a) is firmly fitted to the rim seat part (d) without causing a gap. But the bead outside surface (f) guided by the rim flange (e) starts contacting the upper end of the flange (e) when being assembled into the rim (c), so that a gap (g) is formed between the lower side of the outer side surface (f) and the flange (e).
This gap (g) decreases gradually as the tire runs to be fitted with the outer side surface (f). But along with the decrease of the gap (g), the rim assembly precision varies, and the fitting state of the tire and rim is changed. That is, the balance of the rim assembly state and the distribution state of the force variation are changed. This can lead to imbalance or impairment of force variation. Furthermore, the contact area of the bead part and rim decreases, and a rim deviation occurs between the bead part and rim seat when sudden braking occurs, which disturbs the straightforward motion and lowers the durability of the tire.
In the tire assembled in the rim, meanwhile, although the force variation is checked before shipping, there is a difference in the assembly precision between the rim assembled in the shop and the rim assembled by the user. Therefore, in order to prevent formation of force variation, there has been a keen demand for the development of a tire with such gap even if the tire is assembled in the rim without any particular attention by the user.
As a result of repeated studies and experiments to satisfy the above demand, the present inventors reached the following conclusions.
Formation of a gap between the bead outside surface and rim flange may be inhibited at the time of rim assembling by:
(1) Installing exhaust means capable of communicating between the gap and outside when assembling in the rim; PA0 (2) Installing this exhaust means by providing grooves or ribs on the bead outside surface; and PA0 (3) Setting the inner end of the exhaust means at a position remote from the bead bottom surface in a specific distance in the radial direction. Further, it is found that ribs as the exhaust means are superior to grooves in stability of rim assembly after inflating the internal pressure.
Incidentally, formation of ribs or grooves on the bead outside surface had been shown in the U.S. Pat. No. 4,353,403, but this proposal is intended to prevent swelling of the tire when the tubeless tire is mounted on a wheel having a rim diameter smaller than the design value. In the above proposal, in order to achieve this object, the inner end of the grooves or ribs is extended nearly up to the bead bottom surface. When the composition of this proposal is applied to eliminate the gap caused when assembling on the rim, even the air accumulated in the tire inner space may be forced out.